1. Field of the Invention
The present invention relates to a method for monitoring operation data of an injection-molding machine for quality control.
2. Description of the Related Art
FIG. 2 shows a schematic structure of the injection unit of a hydraulic injection-molding machine. In the figure, reference numeral 1 denotes a mold, 3 a heating barrel, 5 a screw, and 7 a hydraulic cylinder.
The heating barrel 3 is connected to the back surface of the mold 1 via a nozzle 4 arranged at the tip (left end). The screw 5 is installed within the heating barrel 3. The hydraulic cylinder 7 for injection is connected to the rear end portion of the screw 5. A hopper 6 is connected to the portion close to the rear end of the heating barrel 3.
In a charging step, the screw 5 is rotated in the forward direction within the heating barrel 3 by a servomotor (not shown) for charging to introduce resin into the heating barrel 3 from the hopper 6. The introduced resin is heated to melt with agitation, fed toward the front portion (leftward in the figure) of the screw 5, and accumulated in the front portion of the heating barrel 3. Accordingly, the screw 5 is moved backward by the pressure of the accumulated molten resin. After a predetermined amount of the molten resin is accumulated in the heating barrel 3, the screw 5 is moved forward by the hydraulic cylinder 7. In this manner, the molten resin is injected into the mold 1.
Conventionally, the operation data of an injection-molding machine for quality control is monitored in the method described below. First, a reference value and a tolerance limit of the deviation from the reference value are determined with respect to the operation data (such as charging time, injection time, the amount of cushion, charging pressure, injection pressure, and peak pressure). Every time operation data is actually obtained, the deviation of the operation data from the reference value is calculated. When the value of the deviation exceeds the tolerance limit, an alarm is generated to inform the operator that a product may be defective.
The charging time used herein refers to time required for charging a predetermined amount of molten resin into the heating barrel 3. The injection time refers to time from the initiation of injection of the molten resin (from the heating barrel 3 to the mold 1) up to the initiation of a pressure-holding step. The amount of cushion is the forward-moving distance of the screw 5 (or plunger) in the heating barrel 3 during the pressure-holding step. In the pressure holding step, the molten resin is replenished into the mold 1 by moving the screw 5 forward in the heating barrel 3, to make up the room formed by the shrinkage caused by solidification of the resin. The charging pressure is the internal pressure of the heating barrel 3 when the molten resin is charged or the backpressure of the screw 5 (namely, the pressure of the hydraulic cylinder 7) at that time. The injection pressure refers to the internal pressure of the heating barrel 3 when the molten resin is injected from the injection unit into the mold 1 or the backpressure of the screw 5 at that time. The peak pressure is the maximum instantaneous value of the injection pressure during a single injection operation.
The reference value and the tolerance limit of the deviation are fixed. Once they are set, they are maintained at the same value until the operator changes the values.
The reference values are conventionally set in the following manners.
(a) The operator directly inputs a reference value on a set-up screen of an operation board. This method can be employed when the same product as manufactured in the past is to be manufactured and the reference value previously obtained can be applied.
(b) A trial operation is repeatedly performed until nondefective products are stably obtained. At this time, an average value (or the latest value) of the data pieces obtained through the trial operations is set as a reference value. This method can be employed when an appropriate number of products are formed after operation conditions are roughly determined, and then, the reference value is determined while evaluating whether these conditions are proper or not.
The tolerance limit of the deviation is conventionally determined in the following method.
(a) The operator directly inputs a tolerance limit on a set-up screen of an operation board. This method can be employed when the same product as manufactured in the past is to be manufactured and the tolerance limit previously obtained can be applied.
(b) When a recommended value by the device manufacture for the tolerance limit of the deviation has been stored in the operation board (e.g., memory device) in advance, the operator presses a push button to read out the recommended value and set as the tolerance limit of the deviation on the operation board.
The recommended value is stored in the operation board in the following two states.
(b-1) Case where the recommended value is stored as a variable parameter:
The recommended value is stored as a variable parameter in the operation board but not encoded as a fixed value within the control program. In this case, it is generally impossible for the operator to freely modify the recommended value as a variable parameter. However, the service technician of the device manufacturer can modify the recommended value by using a parameter-setting mode without changing the control program.
(b-2) Case where the recommended value is encoded as a fixed value in the control program
The recommended value cannot be changed without changing the control program. Therefore, it is impossible for the operator to change the recommended value. It is not easy even for the service technician of the device manufacturer to change the control program.